While nickel-cadmium batteries and lead acid batteries are already in wide use as secondary batteries, it is desired especially in recent years to develop secondary batteries which have a high energy density and are yet clean with advances in compact information devices such as cellular phones and notebook computers. Accordingly attention has been directed to closed-type nickel-metal hydride batteries which are free from harmful substances such as cadmium and lead and wherein an electrode of hydrogen absorbing alloy is used as the negative electrode.
Nickel-metal hydride batteries comprise a negative electrode of hydrogen absorbing alloy, a positive electrode of nickel, an alkaline electrolyte, a separator, etc. The hydrogen absorbing alloy electrode serving as the negative electrode is prepared by adding a binder to a hydrogen absorbing alloy powder obtained by pulverizing a hydrogen absorbing alloy ingot, and forming the mixture to a shape of an electrode. The hydrogen absorbing alloys developed for use in the negative electrode include those of the Misch metal type comprising a Misch metal (hereinafter referred to as "Mm") which is a mixture of rare-earth elements and Laves phase hydrogen absorbing alloys.
With alkaline secondary batteries, such as nickel-metal hydride batteries, wherein a negative electrode of hydrogen absorbing alloy is used, a gas-phase reaction and an electrochemical reaction proceed at the same time on the surface of the hydrogen absorbing alloy by virtue of the contact of the alloy surface with the alkaline electrolyte. More specifically, in the relationship between the hydrogen pressure and the temperature, hydrogen is absorbed by the hydrogen absorbing alloy, or the hydrogen absorbing alloy desorbs hydrogen (gas-phase reaction). In the relationship between the voltage and the current, on the other hand, application of voltage (charging) causes absorption of hydrogen by the hydrogen absorption alloy of the hydrogen produced by the electrolysis of water, and delivery of current (discharge) causes oxidation of hydrogen to water (electrochemical reaction).
To promote such gas-phase reaction and electrochemical reaction on the hydrogen absorbing alloy surface, a powder of electric conductor is added to the electrode, or the hydrogen absorbing alloy surface is chemically treated, whereas if the attempt to improve the hydrogen absorbing alloy is made singly, various characteristics of the hydrogen absorbing alloy are merely improved partially, and appearance of a new limiting factor hampers great improvements in the characteristics of the alkaline secondary battery.
An object of the present invention is to provide a hydrogen absorbing alloy electrode which is given outstanding battery characteristics by simultaneously ameliorating problems encountered with hydrogen absorbing alloys in the electric conductivity, gas affinity, etc. which are associated with the gas-phase reaction and electrochemical reaction.